This heath was found on moorland near St Keverne, Cornwall. It is considered to be a hybrid between E. vagans and E. tetralix. It grows to 30 in. high and is of dense close habit, the young shoots slightly downy. Leaves closely set in whorls of four, linear, 1⁄3 in. long, furrowed beneath and with very short glandular hairs on the margin. Flowers produced in the leaf-axils towards the end of the shoot so as to form a leafy raceme 1 to 11⁄2 in. long with the shoot protruding through it, or sometimes clustered so as to resemble an umbel at the end. Corolla bell-shaped, 1⁄10 in. wide, rose-pink, with the four lobes erect; anthers brown, included in the corolla; style protruding 1⁄12 in.
The plant grows well in light loamy or peaty soil and in gardens often puts on and can be recognised by a pale green or yellowish (but not unhealthy) tinge. It appears first to have been found about 1860 by Richard Davey, one-time Member of Parliament for West Cornwall, but lost sight of until found again in October 1910 by his nephew, P. D. Williams, after whom it is named and to whom its establishment in gardens is due.
E. × williamsii is a botanical collective name for hybrids between E. vagans and E. tetralix and therefore cannot be restricted to the original clone described above, which should be distinguished as ‘P. D. Williams’. Another member of the E. × williamsii group is ‘Gwavas’, which is dwarfer than ‘P. D. Williams’ and considered to be a better garden plant.